ZnNi and Tagnite Performance on Complex Parts - Corrosion Risk Mapping

1

Alan Rose arose@Corrdesa.com

Impact of ZnNi composition

2

Alkaline LHE ZnNi chemistry and corrosion resistance as a function of wt% Ni. (Bjorn Dingwerth, Atotech, SURFIN (2012).

3

ZnNi thickness & composition with auxiliary anode –typical landing gear strut (simplified)

Very important: Low current density gives low thickness with high Ni%

ZnNi polarization for different Ni content – Corrosion Djinn analysis

4

Matl OCP Galv CorrRate

Aermet 100 -0.730 V

LHE Cd -0.745 V 80 µm/yr

High Ni -0.776 V 77 µm/yr

Med Ni -0.948 V 197 µm/yr

Low Ni -1.14 V 490 µm/yr

On-line Corrosion Djinn® tool gives quick analysis of corrosion rates per new MIL-STD-889D: Low current density -> high Ni content >16%Ni ZnNi too Ni-like → no longer

sacrificial to A100 and 300M <12%Ni too Zn-like → potentially embrittling

Importance of good anode and electrolyte flow design

5

• Thin Ni-rich coating gives no sacrificial protection, exactly where stresses are highest

• Internals do not coat properly• So, internals, blind holes and other complex

shapes require careful anode design (electrodynamics)

• But also careful design of electrolyte flow in awkward geometries (computational fluid dynamics)

Setting up corrosion simulations

6

‘digitally’ create a defect, revealing bare substrate

• Corrosion simulation with galvanic coupling and fluid dynamicsalso allows us to determine the potential effect of protective coating damage

• Allows calculation of corrosion rate• Also allows us to estimation of hydrogen

devolution and therefore SCC risk

ZnNi plated Aermet 100 – with defect

7

For the more noble ZnNi the average potential of the exposed Aermet 100 in the defect is -776mV (just 46 mV below its OCP) compared to -945mV (215mV below OCP) in the case of the less noble ZnNi coating.

What this means is that the less noble ZnNi coating brings down the Aermet potential 215mV (on average) below its OCP, firmly placing the material into the cathodic region and protecting it ‘cathodically’.

Whereas, the more noble ZnNi only brings down the Aermet potential by 46mV, providing little protection.

More noble ZnNi

Less noble ZnNi

8

Impact of Ni varying composition

More noble ZnNi provides less protection… where more strength is required!

So this could be addressed by design and placement of auxiliary anodes